The present invention relates to a miniature vibration motor employed to inform a user of a portable communication terminal device, such as a cellular phone, by vibrations that a signal has been received.
Vibration motors are installed at portable communication terminal devices, such as cellular phones, in order to inform a user by vibrations of the device that a signal has arrived.
Cylindrical vibration motors which have a cylindrical housing and a weight fixed to an output shaft protruding to the outside of the housing and flat vibration motors which have a coin-like housing and a shaft fixed to the housing, wherein an eccentric rotor is provided inside the housing, are the main types of vibration motors.
The rotor in flat vibration motors is eccentric by itself and is rotatably supported on the shaft fixed to the housing. For this reason a configuration is used in which a bearing, a weight and a coil are integrated together with a printed wiring board constituting a commutator substrate in a resin.
For example, Japanese Patent Application Laid-open No. 2002-119915 discloses a configuration in which a bearing is formed by a molded resin constituting the rotor and a configuration in which a sintered oil-impregnated bearing is used as the bearing and supported with the molded resin.
Japanese Patent Application Laid-open No. 2002-28570 discloses a configuration in which a printed wiring board, a hollow commutator coil, a weight, and a bearing are integration molded at the same time by resin molding.
With all the configurations disclosed in those open publications, a flat eccentric rotor is formed by integrating a printed wiring board constituting a commutator substrate, a bearing, a weight, and a coil by resin molding, and they are typically manufactured by resin molding by an injection molding process.
In recent years miniaturization of vibration motors installed on the devices followed the advanced miniaturization of the devices. Bearings such as sintered oil-impregnated bearings are used in the motors so as to rotate the eccentric rotor smoothly despite the reduced size of the motor. Furthermore, because the prescribed vibration quantity has to be ensured despite the miniaturization of the motor, the surface area taken by the coil and the ratio of the weight and size is increased in relation to the size of the motor so as to increase the rotation speed and eccentricity.
In vibration motors, a weight is molded integrally with the rotor. Therefore, if the motor is subjected to an impact, for example, when the device falls, the effect of the impact on the rotor is larger than that in the usual motors because the mass of the motor is increased by the weight.
Because the bearing has a small shape, when it is integrated in a rotor by resin molding, the bearing unit can be fractured by the impact unless a sufficient mounting strength is provided.